Stress indicator for bolts or studs



April 4, 1950 I A.'R. STONE I 2,503,141

STRESS INDICATOR FOR BOLTS 0R s'runs Filed Nov. 12, 1946 v INVENTOR ALBERT :VJGTO STONE H IS TORNEY Patented Apr. 4, 1950 STRESS I NDICATOR FORBOETS OR STUDS Albert Rivington Stone, Baltimore, Md., assignor to Gerotor May Corporation, a corporation of Maryland Application November 12, 1946, Serial No. 709,202

Claims. (Cl. I'm- 311) My application for patent is a continuation in part of my copending application Serial No.

588,650, filed April 16, 1945, entitled, Fastening device, now Patent No, 2,413,797; dated Jan. 7, 1947, and the invention relates to bolts, studs, and the like, and more particularly concerns a method of and assembly for, indicating vor signalling changes in stresses or strains inrsuch bolts, studs or the like.

An object ofmy invention is to provide a mode of determining variations in stresses in bolts, studs and similar fastening devices which have already been seatedinto-the workpiece which is at once simple, effective, inexpensiveand instantaneous in operation.

Another object is to provide an assembly for determining variation in stresses encountered in bolts, studs and other fastening devices which have previously been seated in the workpiece, which are simple, effective, and rapid in operation, small sturdy and compact, of low first cost,

substantially fool-proof, extremely rugged and of.

long useful life.

Other objects and advantages will in partbe.

obvious and in part more fully pointed out hereinafter during the. course of the following description, taken in the light of the accompanying drawings. I

My invention accordingly resides in the several procedural and manipulative steps, and in the several parts, elements and features of construction and as well, in the combination of each of the same with one or more of the others, .the

scope of the application of all of which is more,

fully set forth in the claims at'the end of this specification.

In the drawing, wherein I have disclosed that embodiment of my invention whichI prefer at present, the single figure is a View, partly in longitudinal section, and partly schematic,.showing an.

bines motivated by other fluids, internal combus-,

tion engines of all sorts, fractionating columns and similar devices in oil and other distillation techniques, all demand units which will withstand pressures which were unknown several decades back. Economy and reduction in size is one objective. In other instances, the technique simplycannot be carried effectively to an end in the absence of high pressure techniques. Uniformity in pressure distribution is essential to prevent localized over-stressing and incipient failure at points where such over-stressing occurs.

For one reason or another, therefore, it has recently become important in all fields of high pressure practice to achieve uniform stressing and straining of blots, studs and similar fastening devices when seated in the particular workpiece.

Once seated, these bolts and studs immediately become subject to the internal stressing from the particular fluid or other source of energy restrained within the workpiece. Such stressing, while not necessarily constant, substantially is continuous over a long term of service. Gradualv enlargement of the workpiece is frequently observed. Local over-stressing may occur within the bolt or stud giving rise to a point of incipient weakness. It is important in such instances that the very outset of such failure be called to the attention of the operator. When in this manner, the operator is given sufficient warning it is quite possible that he can set in motion such precautionary steps as will rectify the situation and avoid the threatened failure. Should, however, he wait until the next routine inspection,.quite possibly failure will have already occurred, or else it be too late to set in motion the required steps for correcting the situation.

While it is routine practice in many high pressure techniques to make periodic observations and measurements of pipe line, cylinder head, of whatever particular workpiece is employed, failure may possibly set in at once, intermediate such operations. If only visual or audible signals, or both, were available at the first onset of such incipient failure, preventive steps could at once be enforced and ultimate failure avoided. Con-.

siderable savings both in equipment and in loss ofvv operation are thereby achieved.

Those actively engaged in high pressure techniques have by no means been unaware of the problems attendant upon such practice and have given considerable time and thought to these generalized objectives. To illustrate, the use of electrical strain gauges is well-recognized. For one reason or another, however, because of high firstcosts, complexity of construction, delicacy and fragility of instruments, amongst many other causes, or any combination of one or more of the same, these various expedients heretofore proposed have been uniformly unsuccessful. It was not until my proposed technique appeared on the horizon that for the first time, a truly practical solution became available.

An important object of my invention, therefore, is to avoid in substantial measure the difllculties and disadvantages of the prior art and at the same time to produce an assembly and a mode of operation which gives instantaneous warning to the operator of incipient failure of bolts, studs or similar fastening devices in highpressure systems, all at the very onset of such failure, and this in simple and reliable manner and with the use of inexpensive, simple and sturdy component parts.

In my copending application Serial No. 588,650, I disclose a self-indicating bolt, stud or other similar fastening device wherein the degree of stressing is accurately indicated, so that precisely the desired stressing, straining and length of bolt grip can be accurately achieved. Therein, essentially, I disclose a longitudinally bored bolt, stud or similar fastening device. Fast at the end of the bolt remote from the head end of the bolt, I provide a pin disposed within the bore and at all times free of stressing. This pin either projects a determined distance beyond the open head of the bolt when the latter is unstressed, or else is flush therewith when such condition maintains.

Upon stressing the bolt, the latter is elongated and moves relative to the end of the unstressed pin. The degree of movement upon proper calibration is an accurate measure of the stressing of the bolt.

My present embodiment relates to the instantaneous determination of variation in such stresses, once the bolt, stud or the like has been strained to a predetermined value. Failure due to any one of a number of causes encountered in actual practice can thus be guarded against.

In generalized terms, my invention may be considered as comprising the combination of unstressed means capable of movement relative to a stressed bolt, stud or the like, for indicating the degree of stress within the latter. This, in my present embodiment, comprises an unstressed pin fast at one end of a bore provided longitudinally within a conventional bolt or stud. A snapswitch is provided thereon to give the requisite warning. This snap-switch comprises a sensing means in the nature of an elongated rod which contacts and follows this pin in its movement relative to the stressed bolt or stud. In reality it is the bolt or stud which moves relative to the unstressed pin. This sensing means, rodlike in nature, in the present instance constitutes the plunger of a suitable micro-switch. This latter may be considered broadly as a snap mechanism which is sensitive to a change in the position of the micro-switch plunger in the order of 0.0001 inch. This corresponds to a change of stress of about 3000 pounds per square inch, per inch of bolt grip. I associate signalling means with the micro-switch plunger for signalling to the operator any change in the stressing of the bolt and the strains developed therein. Such signalling means are shown as comprising an electric circuit with a source of electric energy therein and with a signalling means, audible or visual, or both, provided therein.

From the standpoint of method of operation, my invention may be considered as comprising stressing a bolt or stud to a predetermined point as established by a stress-indicating pin provided therein, sensing the position of the pin through a suitable micro-switch plunger, and energizing signalling means upon subsequent variation in the position of the micro-switch plunger as it follows the apparent movement of the unstressed pin relative to the stressed bolt or stud.

And now having reference more particularly to the embodiment disclosed in the single figure shown, I provide a conventional bolt, stud or the like In, similar to that disclosed in my said application hereinbefore identified. This bolt has the usual head H and shank l2, and conventional threading, not shown. I provide a bore l3 longitudinally of the length of the bolt. While it is not essential that this bore extend centrally of the bolt, for simplicity, symmetry and ease in drilling, I prefer to provide such bore, along the axis of the bolt. Again while it is not essential that this bore extend completely through the bolt, it being entirely feasible for it to terminate short of the shank end thereof, I prefer, for ease of manufacture, to have it extend entirely through the bolt, and thus illustrate the same in the drawings. In the bore I3, I provide an unstressed pin 14, fast to the shank l5 of the bore, and free of the sidewalls of the. latter throughout its extent. In the present embodiment during the rest conditions of the bolt, when the same is completely unstressed, the adjacent or free end of the pin l4 terminates exactly flush in nicely machined fashion, with the free or outer surface of the head ll of the unstressed bolt.

As the bolt is seated into the workpiece, whatever the latter may be, a strain is set up by the external stressing at once to elongate the bolt and to cause an apparent retraction of the head of the unstressed pin MA within the bore l3. The extent of the retraction of the end of the pin within the bore by proper calibration, serves as a measure of strain imposed Within the bolt, and when a predetermined retraction has been observed, the operator knows that the desired stress has been exerted and can thereupon discontinue the tightening operation. Uniform stressing throughout the workpiece can be effectively achieved in rapid efficient manner.

I associate with the bolt H) a suitable sensing mechanism to contact and follow the pin I4 in its movement relative to the bolt or stud. After the bolt has once been seated, it is obvious that upon occurrence of incipient failure at one particular point of the workpiece the bolt in that region will tend to become overstressed. This may be due to increase in internal pressure for some reason, as, to illustrate, failure of the excess pressure relief means; upon failure of the metal of the workpiece due perhaps to localized crystal failure; gas flaws or the like; or failure of the metal of the bolt, due to localized overstressing; gassy spots in the metal, or the like. Regardless of the reason for such failure, it is accompanied by localized over-stressing of the bolts, studs or similar fastening devices in the general region of such failure. Increased stressing of the bolt occurs, and further apparent movement takes place between the stressed bolt or stud and the unstressed pin 14. The function of the sensing means, is to follow this pin and to indicate by relative change in position between the end of the pin and the end of the bolt I 0. In the pres ent embodiment, I employ for this purpose a micro-switch 16, of conventional design. Microswitches of this general type are well-known to the art and; are already available. on the market.

An entirelysuitable; and-,-ad equate, form ofv such switch is manufactured for example by the Micro- Switch Division-of the Eirst Industrial-Corporation, havi its main-Iofiice' 1 and factory at- Freebolt III elongates withf thefeffect ofthe. pin I4 retracting within the bore, I3, the micro-switch plunger II willfollow until at such'tjime as it causes break in the allied electric circuit hereinafter to be described. a 5 I -An annular sleeve I6A is; provided on the switch I6, internally threaded at I6B to .adapt the same for mounting on any particular bolt or stud, the elongation of which is tobe determined. Inasmuch as this skirt [6A is of fixed diameter and since various size bolts are encountered in practice, and furtherysinceit is-desirable to make one size of micro-switch adaptable for all of the different sizes or bolt heads, I provide an adaptor I8 between the bolt I0 and the micro-switch I6. It is of course entirely feasibleito provide a different size micro-switch I6 for each size of bolt head. The expense of manufacture in the more limited quantities, however, and as well, the increased ccstsof inventory to maintain the various sizes of micro switch on handin the'lparticular plant greatly increases the costs of such practice-as c'ontrastedto the use of such adaptors. v

' The adaptor I8 cooperates with a threaded extension IIA provided on the outer face'o'f the head, II of the'bolt' I0. Thebore I3 extends through this threaded: extensimand the end I 4A of the bolt I4 terminates flush with the outer extremity .of this extensionflIAwhen the latter is in is unstressed or rest or neutral position. The adaptor I8 is internally threaded as at I8A, in a suitable recess, to receive the threads of the elongated extension I IA of the particular bolt I0. There are as many different sizes of adaptors I8 provided as there are difierent sizes and contoured bolt heads I I in use.

An elongated extension I 83 is provided at the other end of the adaptor I8 externally threaded for cooperation with the threads I6B on the interior of the elongation I6A of the micro-switch I6. Regardless of the diameter of the recessed portion of the adaptor I8 the shank portion I8B thereof is of fixed diameter with threading engaging with that of the extension I6A of the micro-switch I6. In this manner the microswitch I6 is made fast to the end ofv the previously-stressed bolt or stud It. It is threaded down until the micro-switch plunger I1 is in abutting contact with the end I4A of the pin I4 within the bolt III. At this time, a closed circuit is completed across a simple series signal circuit cooperating with the micro-switch I6. The coursing of the current through such circuit may be traced from the lower side of the switch I6 through lead I9 to battery or other suitable source ofv electric supply indifferently alternating or direct current through lead 2| to signalling device 22. This may be a buzzer, bell or other employed. From the signalling; device 22. the'current courses lead 23 back to the-micro-switch I6 thus completing the circuit. Should alternating .current supply be employed,qthe coursing of the current during the next subsequent half-cycle of current flow is exactly the opposite of that just traced.

In use the bolt or stud, is tightened to the de-- scribed, an adaptor 18 maybe provided between the'threaded extension I IA of the bolt-I0 and the shank portion I6A of the micro-switch. While this adaptor of course may be omitted, it is desirable in that it adapts a standard thread on the switch body -I6'to serve different sizes of bolts o'r'studs I0.' Aset screw or the like 24' serves to make the micro-switch I6 fast to the adaptor I8.

1 As has also been pointed out hereinbeforethe combination is highly sensitive to a verysniall change in the tension of the bolt. Whenlocal- 'izedfailure begins either in the workpiece or in the strained bolt itself, under the tremendous oplera'tingpressures encountered, further elonga- ,tio'nof the bolt or stud I0 occurswith consequent apparent recession of the pin'III within the bore I3; Asthe plunger I'I follows "such moti'onit closesa previously opened circuitindicated generally at 26 and energizes the signal device 22,

either visual oraudible. I

.For-the first time, in. rapid, simple ".aiid'ingeni ous manner, it;ha"s been madei'possible for the attendant to receive preliminary warning of'the prospective failure of the system. This warning becomes available to him regardless of whether he'is giving immediate attention to this particular phase of his duties. This distinguishes sharply from the practice heretofore existing, wherein incipient failure could be determined only at such times as actual caliper measurements were being taken. The warning device fits on and combines readily with the standard new and improved stud or the like. Its use is particularly advantageous in inaccessible places, where the attendant is apt to be lax in his observations. Failure could readily occur between measurements and the operator be entirely unwarned until destruction had already taken place.

It is of course, readily understood that it is entirely within the province of my invention to substitute for the working elements, a recording mechanism if desired. Or both may be employed in a typical instance.v

The device is simple, eflicient, rugged, and of long useful life. A minimum number of parts are employed, none of which are moving.

All these and many other thoroughly practical advantages attend upon the practice of my invention. Inasmuch as, once my invention is dissuitable signal, or may be a warning light or the closed, many modifications and adaptations will readily occur to those skilled in the art, and since further many embodiments of the basic inventive concept will likewise suggest themselves, I intend 7 that the foregoing: disclosure beconsidered merely illustrative and notby wayof Iimitation.

1 claim:

1. In combination, a belt or stud adapted to he stressed, an unstressed pin: provided ina longitudinal bore within said bolt or stud' and fast at one end of the bore, means fastened to the boltkor stud adjacent the. other end of said bore comprising a micro-switch having a plunger adapted to contact, follow and sense the movement of said pin relative to the bolt or stud, and an electrical indicating circuit. connected to said micro-switch for indicating excessive movement of said micro-switch plunger.

2. In combination, a longitudinally bored bolt or stud adapted, to be stressed, an unstressed pin within said bore and fast to the bolt or stud at one end" thereof, means fastened to the other end of said belt or stud and including a snapswitch means having a follower plunger adapted to contact andfollow said pin and sensethe movements thereof relative to said bolt or stud, and electrical signal means connected to said snap-switch means for signalling increased stressing of said bolt or stud upon movementof the plunger.

3. Incombination, a longitudinally-bored bolt or stud adapted to be stressed, an unstressed pin Within said bore and fast to the bolt or stud at one. end of the said bore, said bolt or stud having a threaded portion. at the other end thereof, an adapter threadedly engaging said other end of said bolt, a micro-switch threadedly engaging said adaptor and having a sensing, plunger contasting and following said pin in. its movement relative to said bolt 01' stud, and electrical signalling means. connected to said micro-switch for signalling increased stressing of said bolt or stud upon predetermined movement of the microswitch plungerr 4. In combination, a bolt or stud adapted. to be stressed and having a longitudinal bore therein, a continuously unstressed-pin loosely provided in said longitudinal bore and having one end thereof fastened to the bolt or stud with the other end of the pin being free, means fastened to the bolt or stud adjacent the free end of said pin and having a sensing finger adapted to follow the free end of the pin upon stressing of the bolt or stud, and meansassociated with said sensing finger for indicating movement of the latter beyond a predetermined limit.-

5. A system for determining strain variations in previously stressed bolts or studs, comprising in combination, a bolt or stud having a longitudinal bore therein, longitudinal means within said bore and secured at one end thereto and free at the other end permitting relative movement of said bolt or stud with respect to the free end of said longitudinal means in accordance with changes of stress suffered by the bolt or stud, means fastened to the bolt or stud and comprising means for continuously contacting the free end of said longitudinal means within the bore, and means associated with said contacting means for indicating that the relative movement has surpassed a predetermined limit of stress of the bolt or stud.

ALBERT RIVINGTON STONE.

REFERENCES CITED 2 The following references are of record inthe file of this patent:

UNITED STATES PATENTS 

